Non-ohmic or high resistivity contacts, having a contact resistivity higher than 10−4 Ωcm2 are a roadblock to applications of germanium (Ge) despite its high potential for e.g. Complementary Metal Oxide Semiconductor (CMOS) transistors or memory elements. Both CMOS applications on Ge and Ge selector diodes for memory elements need low contact resistivity.
Low resistive ohmic n-type contacts on Ge are difficult to form because of strong Fermi-level pinning on germanium (barrier height is about 0.49 to 0.64 eV for a wide range of metal work function). Ge shows a depinning factor of about 0.05 and a charge neutrality level of 0.09 eV, which means Fermi level pinning close to the top of the valence band.
Secondly, n-Ge contacts are difficult to form because of low activation limits and concentration enhanced diffusion of n-type dopants in Ge. Barrier height lowering by means of thin oxides/nitrides was reported, but the contact resistivity obtained was about 1×10−2 Ωcm2.
Therefore it is desirable to further lower the contact resistivity of the contacts formed on n-type Ge.